<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>包装类</title>
</head>
<body>

</body>
<script>
    // 包装类
    // undefined 和 null 不能有属性和方法

    var num = new Number(123);
    var str = new String('abcd');
    var bol = new Boolean('true');


    // 包装类
    var num = 4;

    num.len = 3;
    // new Number(4).len = 3; delete
    //
    // new Number(4).len
    console.log(num.len);


</script>
</html>
